1. Industrial Field of the Invention
The present invention relates to a television signal scrambling method and apparatus for a CATV system, where many terminals are connected to a center through cables, and picture signals being sent from the center to the terminals.
2. Related Art
Generally, in a CATV system, programs produced in the center and those received over the air are sent to the terminals through the cables. Some programs produced at the center are provided to the terminals for a fee. When such so-called "charged programs" are watched by a subscriber at the subscriber's terminal, a predetermined fare is charged to the subscriber.
To prevent subscribers from illegally viewing the charged programs without paying any fee, the pictures are scrambled at the center ("head end") so that conventional television sets cannot correctly receive the picture signals of the charged programs.
On the other hand, for the subscribers who are authorized to view the charged programs, a descrambling apparatus for restoring the scrambled picture signals is installed.
Various types of the scrambling and descrambling methods have been used or suggested. One method currently in use is the so-called GSS (Gated Sync. Suppression) method. According to this method, the horizontal and vertical synchronizing signal portions of a television signal are compressed by several dB such that the conventional television set receiving such a signal cannot synchronize horizontally and vertically. However, at subscriber's terminals the horizontal and vertical synchronizing signal portions of the television signal are expanded to their former levels so as to restore the original television signal.
FIGS. 7(a)-7(c) illustrate the result of a scrambling operation using the GSS method. The Figures show signal wave forms of the VBI portion of a television signal following the last portion of horizontal synchronization, H.
FIG. 7(a) illustrates a portion of a conventional television signal which has been converted into an intermediate frequency picture signal, IFv. As shown in FIG. 7(b), at the transmission side, that is, at the center, the horizontal synchronizing signal (H) and the vertical blanking interval (VBI) in the intermediate frequency television signal IFv are compressed a particular amount, for example, approximately 6 dB or 10 dB. As shown in FIG. 7(c), a key signal, representing a compression timing of the horizontal synchronizing signal, is superimposed on an intermediate frequency FM audio signal IFa at the center. The resultant signal IFa and the intermediate frequency television signal IFv are up converted to an RF signal and sent via the cables to the terminals.
At each terminal having a descrambler, the intermediate frequency audio signal IFa is extracted from the received RF signal, and the key signal K is detected from the intermediate frequency audio signal IFa. The timing of the detected key signal K is used to restore the horizontal synchronizing signal portion of the television signal by expanding the same by approximately 6 dB or 10 dB. In addition, the absence of the key signal is detected and the VBI portion is expanded for approximately 6 dB or 10 dB so as to restore the received signal to the former television signal.
When television signals are scrambled according to the aforementioned scrambling method, non-authorized subscribers can descramble them relatively easily, thereby illegally viewing the charged programs.
For example, at each horizontal blanking interval (HBI), the key signal is AM modulated and then superposed on the audio signal. If one detects the key signal, which is accomplished with an AM detector, and passes the detected key signal to an integration circuit or the like, the synchronizing signal of the VBI portion can be identified. Subsequently, by operating a free running timing generator in synchronization with the synchronizing signal, the amplitude of a pulse that the free running timing generator generates is adjusted in accordance with the integration level of the synchronizing signal so as to descramble the received signal.